Effect of Resin Cement Color on the Final Color of Lithium Disilicate All-Ceramic Restorations.

Objectives
Obtaining an adequate ceramic thickness to mask the substructure color is not always feasible, and appropriate use of a cement may be the only solution. This study aimed to evaluate the effect of the color of Variolink II resin cement on the final color of lithium disilicate glass ceramic restorations.


Materials and Methods
In this in-vitro study, 90 discs of IPS e.max Press ceramic were evaluated. The ceramic discs were cemented to composite and amalgam blocks. The effect of the cement color and substructure on the final color of ceramic was analyzed by calculating the color change (ΔE) value using a spectrophotometer. Data were analyzed via three-way analysis of variance (ANOVA) and Tukey's test.


Results
The cement color had a statistically significant effect on the final color of ceramic (P≤0.001). The white, yellow, and translucent cements caused the highest color change (ΔE=4.558, 3.308, and 2.649, respectively). The effect of composite substructure and the yellow cement on the final color was less prominent compared to other combinations of cement and substructure (ΔE=2.043). The white cement over amalgam substructure showed the greatest effect on the final color (ΔE=4.890). The ΔE in HO group was less than that of other combinations (P<0.05), and the greatest ΔE was reported in MO group with the white cement (ΔE=6.255).


Conclusions
The final color of the restoration is influenced by the cement color. Therefore, when IPS e.max Press is used over a metal core, it is recommended to use a cement with an HO ceramic.


INTRODUCTION
Optimal color match of restorations with adjacent natural teeth is a major challenge in dentistry [1,2]. All-ceramic restorations (with no metal substructure) have a higher translucency; thus, they are suitable for use in the esthetic region [3,4]. Among different ceramic systems, lithium disilicate glass ceramics are highly popular due to their adequate strength (350-450 MPa), optimal bond to dental structures, easy fabrication process (the lost wax technique in comparison with the layering technique), and excellent esthetic properties [5,6]. Previous studies suggest that the thickness of ceramic should be at least 2 mm in order to mask the effect of the underlying discolored tooth or the abutment color on the final color of the restoration [7][8][9]. In many clinical cases, achieving a 2-mm axial reduction www  is not possible without encroaching the pulp and compromising the strength of the remaining tooth structure. In such cases, over-milling is often performed, which may compromise the durability of the restoration [10]. When achieving an optimal ceramic thickness is not feasible, using a cement with an appropriate thickness and color might be the only available solution to mask the color of the substructure and its effect on the final color of the restoration [10- 16]. Spectrophotometers are among the most accurate tools for tooth color measurement in dentistry [17][18][19]. The color change (∆E) value is often used to compare two different colors and is calculated using the following equation: ΔE=(ΔL2 + Δa2 + Δb2)½. If ΔE is bigger than one, the color difference is significant and visible for at least 50% of the observers. In case of ∆E>2.7, the color difference is not clinically acceptable [20]. The effect of the resin cement color on the final color of ceramic restorations has been previously evaluated; however, no applicable guideline is available regarding the use of different cement colors in the clinical setting [11,12,14,15]. Therefore, considering the introduction of new ceramics with different translucencies, more studies are required to understand the effect of the substructure and the cement color on the final color of ceramic. The aim of the current study was to evaluate the effect of the color of Variolink II resin cement on the final color of IPS e.max Press ceramics to obtain the best combination in use of all-ceramic restorations in terms of the color match.

MATERIALS AND METHODS
This in-vitro experimental study evaluated the effect of the cement color on the final color of lithium disilicate glass ceramics. Ninety specimens were fabricated for the assessment of high opacity (HO), medium opacity (MO), and low translucency (LT) cores, three cement colors (white, yellow, and transparent), and two types of substructure (amalgam and composite). The specimens were randomly divided into 18 groups of five using a  For this purpose, wax and acrylic patterns with the shape and size of the desired discs were required. These discs had to be 12 mm in diameter and 0.6 mm in thickness (according to the manufacturer's instructions regarding the minimum required thickness). For the fabrication of these acrylic patterns, four metal stops measuring 3×3 mm with the thickness of 0.6 mm were fabricated to be placed between two glass slabs creating a 0.6-mm space. Next, according to the manufacturer's recommendations, a paste was prepared by mixing the monomer liquid and polymer powder (Pattern Resin; GC. Corp., Tokyo, Japan), which was placed between the two glass slabs. After complete setting, a 0.6-mm thick acrylic sheet was fabricated. For the fabrication of monolayer LT discs with the thickness of 1.2 mm, stops with the same thickness were used. Next, the discs with the same diameter were cut out of the acrylic sheet using a trephine bur (Biomet 3i, Palm Beach, FL, USA; Fig. 1). These discs were sprued and used as a pattern for the fabrication of ceramic cores. After casting, the discs were separated from the sprue using a separating disc (Ivoclar Vivadent, Schaan, Liechtenstein) and were then ground using the grinder discs recommended by the manufacturer. The thickness of the discs was adjusted using a digital caliper (Maxwell Digital Caliper, Ohio, USA). These stages were also performed for HO and MO cores. For the LT ceramic, the same procedures were followed to fabricate a 1.2-mm thick pattern to obtain monolayer LT ceramic discs. Next, in order to add a 0.6-mm thick veneering layer over the core, a mold was used, which was made of a metal sheet measuring 30×30 mm with the thickness of 1.2 mm. There was a 13mm-diameter hole at the center of the mold such that the core discs could be easily placed in it. The cores were placed in the hole, and the IPS e.max Ceram layering (veneering) was applied. Excess material was carved. To compensate for porcelain shrinkage, the porcelain was applied over each disc twice and was baked to obtain a 1.2-mm thickness. The discs were then finished using Diagen-Turbo-Grinder discs (Ivoclar Vivadent, Schaan, Liechtenstein) to create an equal thickness. A roundend cylindrical bur (S835R/012, SwissTEC, Switzerland) was used for creating a groove measuring 3×1.2 mm, perpendicular to the periphery of the disc, and then, all double-layer MO and HO and monolayer LT discs were auto-glazed according to the manufacturer's instructions. In this study, amalgam (Sinaluxe, Shahid Faghihi Co., Alborz, Iran) and composite (Tetric Ceram®, Ivoclar Vivadent, Schaan, Liechtenstein) substructures were fabricated as follows: a metal disc with the thickness of 1.3 mm and the diameter of 13 mm with a groove perpendicular to the periphery (measuring 2.5×1 mm) was glued to the bottom of a plastic container (at the center) measuring 30×30 mm with a 5-mm depth using liquid glue. A paper disc with the diameter of 10 mm and the thickness of 30µm was glued to the metal disc (at the center). The amalgam was then condensed around the metal disc as recommended by the manufacturer until the plastic container was filled with amalgam. Similarly, the composite was incrementally applied to the plastic container around the disc such that the container was filled with composite. After complete setting, the substructures were removed from the container. The presence of a metal disc created a space inside the composite and amalgam for subsequent placement of ceramic discs. The paper disc on top of the substructures created a 30-µm space for the cement. The groove at the peripheral margin of the metal disc created an appendage in composite and amalgam blocks to match the groove in the ceramic disc. This was done for reproducible positioning of ceramic discs. Also, the same area of the discs was subjected to spectrophotometry (Fig. 2). Variolink II resin cement (Ivoclar Vivadent, Schaan, Liechtenstein) was used for cementation. The ceramic discs were cemented to composite and amalgam blocks. The effect of the color of amalgam and composite substructures and the cement color on IPS e.max Press ceramic was compared with that of the control group (A2 shade of ceramic, glycerin instead of cement) using a VITA Easyshade spectrophotometer (Vita Zahnfabrik, Bad Säckingen, Germany). In order to fix the disc in front of the spectrophotometer, a fixing tool was needed, which was designed by fixing a mounting jig base of a Bio-art articulator (Bio-Art, São Carlos, Brazil) in stone plaster type III (GC America Inc., Alsip, IL, USA) which was placed in a dental aluminum tray. Amalgam and composite substructures were attached to the fork of the spectrophotometer using a plastic plate (Fig. 3). The color change (ΔE) of the test groups and the control group was calculated according to the following formula: ΔE=(ΔL2 + Δa2 + Δb2)½, where ΔE is the color change, ΔL refers to change in lightness (L parameter), Δa refers to change in a* color parameter (indicative of greenness-redness), and Δb refers to change in b* color parameter (indicative of blueness-yellowness). Data were collected and analyzed by SPSS version 18 software (SPSS Inc., Chicago, IL, USA) via three-way analysis of variance (ANOVA) and Tukey's test with the level of significance set at 0.05.

RESULTS
Three-way ANOVA was used to assess the effect of the study variables on the final color of the restoration. Table 1 presents the results of this test. As shown, the interaction effect of the substructure, cement, and ceramic on the final color of the restoration was not significant (P=0.217); however, other interactions were significant (P<0.05). The results showed that the mean ∆E for the three ceramic groups after cementation with three different cement colors on two different substructures was 1.807, 4.353, and 4.355 for HO, MO, and LT ceramics, respectively. Since the effect of different cement colors on the final color of the restoration was significant (P<0.001), Tukey's test was used for pairwise comparisons of the effect of the cements. Considering that the interaction effect of the substructure and the cement on the final color of the restoration was found to be significant (P<0.001), Tukey's test was applied for pairwise comparisons ( Table 3). As shown in Table 3, the mean ∆E for the combination of composite substructure and the yellow cement was less than that of other combinations (∆E=2.043). The application of the white cement along with amalgam substructure yielded the highest ∆E (4.890). Based on the results of this study, the interaction effect of the cement and ceramic on the final color of the restoration was significant (P=0.001); therefore, Tukey's test was applied ( Table 4). As shown in Table 4, the ΔE after cementation in HO group was less than that of other combinations (P<0.001), and the greatest ΔE was reported in MO group with the white cement (∆E=6.255).

DISCUSSION
In order to fabricate an esthetic restoration, clinicians and technicians should take into account all the factors that affect the final color of the restoration. In the current study, the effect of the cement color on the final color of ceramic with metal and composite substructures was evaluated. The results of three-way ANOVA showed that the cement color had a significant effect on the final color of ceramic (P<0.001).
The effect of different IPS e.max Press ceramic ingots (HO, MO, and LT) on the final color of the restoration was also studied. Based on the manufacturer's claim, an HO ceramic with the thickness of 1.2 mm is capable of masking the silver color of amalgam. In our study, the color of the MO ceramic was significantly influenced by the silver color of amalgam.  reported that the curing process of the cement influences the final color. Therefore, the results of studies in which the cement is properly cured are scientifically more reliable since they better simulate the clinical setting [23]. The third factor is the difference between the color of the main cement and the try-in paste. Although the manufacturers produce try-in pastes that are compatible with the main cement, El-Hejazi and Alsufayyan [24] showed that the final color of the restoration changes after using the main cement instead of the try-in paste. By comparing the mean value of ∆E associated with the combination of substructure and cement, it can be concluded that when using a composite substructure and a yellow cement (ΔE=2.043), the final color of ceramic is influenced less by the cement in comparison with other combinations. Also, this effect was most prominent when using the white cement and an amalgam substructure (ΔE=4.890). The comparison of the mean ΔE value in use of ceramic and cement showed that the mean ΔE of the white cement and the HO ceramic was the lowest compared to other combinations; whereas the greatest ΔE in use of the white cement (ΔE=6.255) and the translucent cement (ΔE=3.163) was related to the MO ceramic core. However, for the yellow cement (ΔE=4.038), the greatest ΔE was reported when an LT ceramic core was used.

CONCLUSION
According to the results of the present study, we may conclude that when an MO ceramic core is used with a dental substrate or with a substructure of a suitable color, the color change resulted from the application of the cement is more prominent.
In case of discoloration of the dental substrate or improper substructure color, the optimal final color should be achieved by choosing the proper cement color. It seems that the combination of a white cement and an HO ceramic core would be a proper combination to prevent the adverse effect of the substructure color on the final color of the restoration.